Alternating current hoist control



Filed Dec. 8, 1943 2 Sheets-Sheet l Hoist Lower WITNESSES:

I I INVENTOR ay/ g6 M l/mg? fh zcfkelfiazrz.

ATTORNEY c w. R. WICKERHAM 2,386,580

ALTERNATING CURRENT HOIST CONTROL Filed Dec. 8, 1943 2 Sheets-Sheet 2Conic/afar ATTORNEY human. 9, 1945 UNITED STATES PATENT OFFICEALTERNATING CURRENT HOIST CONTRGL William R. Wickerham, Swlssvale, Pa.,aaslgnor to Westinghouse Electric Corporation, East Pittsburgh, Pa., acorporation of Pennsylvanlo Application December 8, 1343, Serial No.513,351

18 Claims.

My invention relates to olternatinswurrent op erated contsol systems forhoist meters on. cranes,

mine hoists and other hoisting and elevating de-' vices and. deals withsubject mottes related in While evoiomo solvents tlon.

A. see

or substoeticlly reducing" clls= types of energizeobject all t gtheselines is which ea orcls developing high "o oull-out toze ue highhoisting and high to lo speeds reletlvely low eus= rent input but emceeP tomue to relatively slight values when setti the msstet controllerfol" lowest lowering speeds.

Another object of the ir ention relates espe ciolly to hoist coma-clsyste v in which an clter= noticecurrent hoist motor, s ell as awouzzdrotor motor, has on exterior secoio eooigzqoelwith resistors forcoon tcccoue no and contains control to? select vely elemnoting" partsor oz 1 ese to provide several 0i 'gmgressive acceleration or cleceloration. the known systems of t so, just as many resistance provided inthe sec omlitry' circuit; a corresponding member of secomlary-controlrelays, as the desired num= lost of acceleration steps or correspondingcontrol positions of the octueteel master con troller. contrast thesetothe inventloo aims at providing mtfitbstep o1" Wi18=33tllg occolorationcontrol While z'eclucing the smelter of secondary resistance steosenciapper't' seo= ondozry contector eouipmeot below that heretoforenecessary. For instance, the invention will achieve a control coveringthe ranges of, say, tons to six customczw control steps while using onlytwo resistance steps in the exterior rotor circuit" Still another objectof my invention, also related to occeleretlon control by means of comtrolled resistors in the tote": circuit of. on otter: natingmurrenthoist motor, to prevent or sub or rotor c cuit HSSUED (c1. two-soc; SW 30 194? else the changes of torque associated with anoperalter-controlled change of acceleration. That is,

when advancing the master controller from one to another control pointfor changing the re s sistonoe in the circuit, the torque tends tochoose suddenly; cool the intention, in the gases ent aspect, ms atreducing" or cushioning this undesirable fuslct'loolng.

is also an object lo'veotion to -it in cl system on costs ed byeutometl= tool by e. solenol-l c1 Moe poetical csoly controlll operated.sees it is on object more out; rendering e 33 also responsive to motorso The l; ten

control systel let: he is? fi -torque owrves com one 3 alts lattes- 2'toe wounoi rotor tor Circuit whose 1 are provided with erior resistors sed and 9.2, resoeetively. The rotor 5 drives shaft 8 which is coupledToy c gees cox with hoist to o cereted. i-l 'lZtVSlii 3 n t "d iscoto'tt'ollecl gear b so as to engage one end gsosltloo. move-Tolecontact member: 8 of hoist limit switch HS and in the other position themovable member of a lower limit switch LS, depending upon the directionof rotation of the motor HM.

A mechanical brake BR is connected with the drive shaft 6 and iselectrically energized by means of a magnet coil 6|.

A hoist relay RI has its contacts III, ZII and 3! I controlled by arelay coil I I so that when the relay is energized the motor HM isenergized for running in the hoisting direction. A lowering relay R2having contacts H2, ZIZ and 3I2 controlled by a relay coil i2 serves toenergize the motor HM for operation in the lowering direction.

A saturable reactor SR has its magnetizable core 5! provided with analternating-current coil 52 and a premagnetizing control coil 53. Coil52 is series arranged in the phase connection 3 of the hoist motor andis wound in two sections which are so located on core 5i so thatvirtually no transformer effect is produced on coil 53. A reactorcontrol relay R3 is provided to short circuit the reactor SR by theclosure of relay contact 3B3 when the appertaining relay coil I3 isenergized. When contact Sit is closed, the reactor is ineifective sothat the motor HM is energized for full three-phase operation anddevelops a corresponding three-phase torque characteristic. When contact3R3 is open, the current supplied through the phase connection 3 dependsas to magnitude on the effective reactance of coil 52. This reactance iscontrolled by the energization supplied to the reactor control coil 53.That is, if the current in coil 53 is low, the core at is magneticallyunsaturated and hence the reactance of coil 52 at a correspondingly highvalue. On the other hand, when coil 53 is magnetized to such an extentas to saturate the magnet core 59, the reactance of coil 52 is at aminimum. The range of variable reactance thus obtained is so chosen thatthe current in phase connection 3 is only slightly reduced when thereactor is saturated so that the motor HM is then energized forapproximate three-phase operation, while when the reactor is unsaturatedthe effective reactance is so large that the current in phase connection3 and the voltage across the respective primary are suificientlysuppressed to operate the motor HM in approximate single phaseoperation. The means for effecting the just-mentioned reactor controlwill be described hereinafter.

The electromagnetic brake BR has its coil BI connected to a rectifierbridge 52 through the contact N5 of a brake relay R5 Whose operatingcoil is denoted by l5. The rectifier bridge 52 is controlled by fourrelay contacts II-il, 2M, 3M and M4 appertaining to a relay Rd. When thecoil I4 of relay R 5 is deenergized, contacts lid and 3 I4 are closedand connect the rectifier bridge to the mains 2 and 3. Consequently, inthis position and assuming that contact II5 of relay R5 is closed, thebrake coil BI is energized from a substantially constant current sourceso that the brake is fully released. When coil Id of brake relay R4 isenergized, contacts Il i and'3I4 are opened and contacts H4 and 4Mclosed. As a result, coil 6| is now connected through a calibratingregister 64 with two leads 80 and 90 of the rotor circuit and thereforeenergized by a voltage sumciently low to reduce the braking effort atzero speed Without releasing the brake entirely. Since the voltage ofthis circuit depends on the speed of the motor, the braking forceapplied by the brake is now dependent on the motor speed. That is, whenthe speed is low, a higher voltage is applied to coil 6!, thus producinga lesser braking effect than when the motor speed is high and thesecondary rotor voltage correspondingly reduced. The brake relay Rd hasfour additional contacts 5M, 6M, H4 and 814 whose purpose will bediscussed in a later place.

Two relays R6 and R1 are provided for controlling the resistance in therotor circuit of the hoist motor. Relay R6 has contacts MG and 2 I6which, when closed, short-circuit all of the resistors in the rotorcircuit. A further contact alt serves an auxiliary purpose and isoperated together with contacts H6 and ZIB by a relay coil It.Similarly, the relay R! has contacts Ill and -2Il for shorting theresistors i2, Bland 92 of the rotor circuit and an auxiliary contact 3Wcontrolled by a relay coil i'l.

Two auxiliary relays R8 and R9 serve to control the operation of thehoist relay Bi and R2 under certain operating conditions, as will beapparent hereinafter.

The system contains also two timing relays Ti and T2 which arecontrolled by the operation of the secondary relays R5 and Bi. Each ofthese time relays has a main magnetizing coil denoted by It and 20,respectively, and an oppositely acting neutralizing coil denoted by I05and 205, respectively. The neutralizing coils have a smaller number ofampere turns than the main coils and serve to afiord a definite timedelay. Each relay is further provided with a short circuited windingconsisting, for instance, of a copper sleeve or ring I36 or 235 forincreasing the time constant. Relays of this type are known as such anddo not form a feature of the invention proper, although the presence ofsuch timing means is preferable in systems according to the inventionfor assuring a desired time limit between the subsequent operation ofrelays R6 and R1, or vice versa.

When energizing the supply circuit, the terminals G and D of the seriesconnected neutralizing coils M0 and 209 are supplied with current sothat these coils remain effective during the operation of the system.Due to the dimensioning of these neutralizing coils and since theireffect is in opposition to the active operation of the timing relays,the energization of the neutralizing coils does not effect the openingof the appertaining relay contacts. The main coils ii! and 2E] of thetiming relay are connected at E and F to a current source when thesupply mains I, 2 and 3 are energized. However, the circuit of coil it)extends over contact 3m of relay R6 so that relay TI closes its contactH0 only when the relay R6 is energized. Similarly, the main coil 26 oftime relay T2 is active only when contact SI? of relay R7 is closed andcloses contact 2"! when I deenergized.

In order to control the hoist motor HM for hoisting and loweringoperation at different speeds, and for controlling the reactance ofreactor SR in order to regulate the speed torque characteristic of themotor, a master controller MC is provided. This controller contains aplurality of contact fingers, such as those denoted by 43, whichcooperate with a number of contact segments mounted on a common carrieror base 40. The two upper segments denoted by 4| and 42 serve to controlthe energization of the reactor control coil 53. To this end, coil 53 isconnected to the master controller through a rectifier bridge 56 whichis fed from points A and B of supply mains 3 and 2, respectively. Thecontact fingers appertaining to segments 4| and 42 are cona,ssc,sso

The brake control relay R3 has a coil I3 connected through conductors.IN and I32 between the master controller and a push button PB which inturn is connected to a conductor I32 leading to point C of main 2. Therelay coil I3 is energised only when the push button PB is actuated andwhen at the same time the master controller establishes a connection oreither conductor IN or I32 with the conductor I33 leading to point A oimain 3. 1 I

The operation of the system will be understood from the followingdescription of the relay operations, and the functions performedthereby, when the master controller MO is moved from the "01! positioninto the hoist and lowering positions denoted in Fig. 1 by numerals Ithrough 3, and I through 1, respectively. 'In the following description,reference is also had to the sequence chart of Fig. 2 which indicatesthe closure 01' the relay contacts in relation to the eight hoistingpoints and seven lowering points of the master controller.

According to Figs. 1 and 2, none of relays RI through R3 are energizedwhen the master con-v troller is in the "off" position. Hence contacts3", 3H and M3 are closed. As mentioned before, neutralizing coils I30and 200 of the time relays are energized. Main coils I and 20 of timerelays TI and T2 are also energized through E, III, 3I3. F and E, 23, 3|1, F. Hence contacts H0 and 2H! are open. The brake BR is deenergizedbecause contact II! is open. The saturable reactor SR is connected inthe motor circuit since contact 3 I 3 is open.

Starting from this ofl position, the following is a brief step by stepdescription 01' a hoisting and a lowering operation.

Holsrmo OPERATION Step 1 In this position of the master controller MC,resistor 33 is connected to rectifier set 38 by segment II of controllerMC and has its full resistance in series to the reactor control coil 53.Hoisting relay RI is energized over B, H, HS, I33, MC, I33, A. ContactsIII, 2 and 3 close and energize motor HM for hoisting.

Relay R3 is energized through C, I32, I3, I35, MC, I33, A, and closescontact H5. Hence. brake coil 3| is'connected to rectifier 62 and, R3not being energized, is excited from the line to completely release thebrake. Relays R3, R3 and R1 remain deenergized. Hence, SR is eflectiveand the total resistance in the rotor circuit is also efi'ective. Sincethe premagnetization of SR by coil 33 extends over the full value ofresistance 35, SR is not saturated and its reactance at a maximum. As aresult, the motor is energized for approximate single-phase operation,that is, for hoisting at low speed and low torque. The speed-torquecharacteristic now effective is exemplifled by the positive branch ofcurve C1 in Fig. 4. 1

Step 2 position, the uppermost sec- 53 is shorted by segment 3i and Inthis controller tion of resistor causes the magnetization of SR toincrease and rectifier bridge" Steps 3 through 6 While moving the mastercontroller through positions 8, 4, 5 and 6, the effective resistance 01'series resistor 33 is progressively diminished andthe reactance or SRincreased, thus increasing the current in phase connection 3. As aresult,

the speed-torque relation approaches progresssively a multi-phasecharacteristic. At step 6 the resistance of resistor 33 is at a minimumor zero so that the full saturating current is applied to control coil33. In this condition, the magnet core II is saturated and hence thereactance or SR at a minimum. The speed-torque characteristic obtainedat step 6 is represented by curve C: in Fig. 4. This characteristicapproaches that of a three-phase energization of the motor. In theintermediate control steps, the torque characteristic then eilectiveresumes a corresponding position intermediate curves C1 and Cr.

Step 7 Switching from step 6 to step 7 effects no further change in .theresistance or 33. However, R1 is now energized through A, I33, MC, I31,I1, I32, C and shorts 12, 32, and 92 at H1 and 2" for acceleration. Themain coil 23 of T2 is deenergized by the opening or contact 3". Contact21o closes with delay and thus prepares relay R8 for its subsequentoperation. The removal of resistors 12, 82, 32 increases current inputto the motor. as well as the torque developed and tends to causacceleration to curve C1. The torque does not change instantaneously toC: since the increased current through SR results in a'temporaryreduction in voltage on the motor terminal until such time as theaccelerating current has disappeared. Thus, the saturable reactorproduces also a cushioning efl'ect resulting in gradual change in thespeed torque conditions.

Step 8 The current and torque reactions described under '7 above arerepeated with the result that speed-torque characteristics as shown bycurve C4 are attained. More in detail, in addition to the adjustmentsobtained at step 7, the relay R3 is now energized over C, I32, 2IIi, I3,I33, MC, I33, A, and shorts resistors 1|, 8|, 9| at H3 and 2I3. Theopening of contact 3I6 eflects the deenergization of coil l0 so thatrelay TI closes contact H0 with delay. This brings relay R3 in over C,l32, H3, I3, MC, I33, A. Contact M3 is closed thereby shorting thereactor SR. As a result, the motor HM operates now in iullthree-phaseenergization with a correspondingly higher pull-out torque asrepresented by curve C5 in Fig. 4.

LOWIRING Oesaanon To start the lowering operation the master controllermay be moved directly through the dead positions 1 to 6, to the 7thposition where acceleration to full speed rapidly takes place.Subsequent retardation in speed results when the controller is returnedthrough positions 6-5-3- 32-|. Thiscourse of actuation is considered inthe following abbreviated description oi a lowering operation.

' Step 7 Relay R2 is energized through B, I2, l3l, MC, I33, LS, A, sothat 2, 2l2 and H2 are closed for running HM in the lowering direction.Relay R1 is energized through C, I32, I1, I31, MC, I33, IS, A and shortsresistors 12, 82, 32 at H1 and 2. C011 20 of T2 is cut oil. at 3" sothat shorts resistors II, 8|, 3| at H6 and 2I3. Hence the rotor circuitcontainsno exterior resistance.

The opening contact 3I8 releases coil I3 of time relay TI so thatcontact III) is closed with delay. Coil I3 of R3 comesin over H andcloses contact 3I3, thus shorting SR out of the primary circuit. RelayR3 comes in through 0, I32, l8, 3, MC, I34, LS, A, and closes contact H3thus energizing relay R3 through I32, I9,- 3, 140, I34, 15, A. Relay R5is energized over I32, II, MC, I34, LS, A.

As a result, the motor operates for high-speed lowering under power andthe reactor SR is ineflective so that'the motor develops full three-.phase torque.

. Step 6 After passing from position I to position 6, the 3 controllerMC interrupts the circuit between I8 and I34 so that R3 falls off,closing 2 l3 and opening H8. Coil I2 of R2 is likewise disconnected atMC and opens II2, 2I2 and 3I2. Relay R9 remains in because coil I3 isenergized from I32 through the self-sealing contact H9, and energizes RIthrough B, II, HS, I38, 2I8, H3, MC;

the negative branch of curve C2 in Fig. 4.

Steps 5 through 1 During these subsequent steps, the relay and contactconditions of the system remain un- .changed. Merely the resistance of55 is stepwise dimensioned by segment 42 of MC to increase themagnetization of SR in order to change from approximately three-phasetorque to approximate single-phase torque. At step 1 the resistance of55 is at a minimum or zero and hence SR set for full saturation. Duringsteps 6 and 1, the torque characteristic assumes positions between thenegative branches of curves Cs and C1 in Fig. 4.

It is assumed in the above description of the function of the mastercontroller that the push button PB remains in the illustrated openposition in which the relay R4 is ineffective and hence connects thebrake BR to the line for constant braking force. However, the operationof the master controller aifects also the condition of readiness of thepush button control. In this respect, it will be seen from Fig. 1 thatthe coil I4 of brake relay R4 is ready for energization only when themaster controller is in the o position so that the push button PB isconnected through coll I4, lead I6I and the master controller with leadI33 attached to point A of main 3. Due to the construction of the mastercontroller. the push button PB remains ineffective during hoistingoperations as well as during the high acceleration steps of loweringoperations. However, once pushed in the off position of the mastercontroller, the push button PB can be kept effective over self-sealingcontact SH and lead I62 when the master controller is moved from ofl'into position 1 for slow lowering operation, but is cut oil at MC whenMC is moved farther into The positions 2 to 7." When the mastercontroller is moved out of the "oflP-position before previouslyactuating the push button PB, the relay coil I4 cannot be energized. Theoperation of relay R4 has the effect changing the excitation of brake BRfrom 'the line to the secondary or rotor circuit, of the hoist motor HM.As a result, the'braking effect, when under push button control, isdirectly proportional to the motorlowering speed and is especiallyeffective at high motor speeds. When controller MC is moved from "on" tolowering position 1 while button PB is kept pulihed, the energization ofrelay R4 has the efle'ct of actuating relay R2 through 13, I2, I3I, 1I4,I43, MC, I33, A and applies lowering power to the motor primary. At thesame time, contact II4 of R4 inserts resistor 34 into the premagnetizingcircuit of the reactor RS with the result of applying a fixed degree oflowering torque at zero speed. This torque falls to zero at hronousspeed, and this coupled with increasing brake torque serves to hold themotor at greatly reduced speed at light or no load as well as withnormal load.

Reviewing the system and its operation, it will now, be apparent that amulti-step control of the speed torque characteristic of a hoist driveis obtained which, by virtue of the above-described saturable reactorfeature, requires only a small number of resistance stepsin the rotorcircuit of the hoist motor.; It will also be seen that the system.aftords obtaining high multiphase torques and at the same time oflergthe advantage of low torques at low speed lowering also by virtue of theabove explained use of a saturable reactor. Furthermore, the system asdescribed in the foregoing prevents sudden changes of torque whenchanging from one to another speed torque characteristic and this isalsti:J achieved with the aid of'the saturable reac r.

This utilization of a single controllable impedance device for thejust-mentioned diflerent hoist control functions is one of the salientadvantages-of the illustrated embodiment and contributes essentially toattaining the above-mentioned functions by relatively simple and com- Imet devices. a

' favorable to a simple design and operation of Another advantage ofsystems according to the invention is the fact that the regulationbetween three phase characteristic and approximate single phase (lowspeed low torque) characteristic is obtained without changing theinternal connections of the hoist motor. That is, a standard motor,having its primaries permanently connected with each other in delta orstar connection, can be employed since the variation between thesecharacteristics is achieved by externally connected control means, i.e., by varying the voltage distribution through a range which includes asubstantially balanced three phase energization and a highly unbalancedor approximate single phase energization.

It will further be apparent that'the reactor control, the control of thecounter torque operation, and also the control of thepush-buttonactuated braking operation are effected by means I of asingle master controller, an advantage also the system. By combining thepush button with the master controller, in particular, by placing thebutton in the handle or crank of the master controller, both are ineffect capable of one-hand operation.

Since obvious various modifications on the basis oi the abovedlsclosure'and in accordance with the gist and scope of my invention areavailable to those skilled in the art, I wish this specification to beunderstood as illustrative and not in a limiting sense.

I claim as my invention:

1. A control system comprising an alternatingcurrent motor, multiphasecircuit means for energizing said motor selectively for operation inboth directions, saturable reactance means formin part of said circuitmeans and including premagnetizing control windings, variable energizingmeans connected to said windings for changing the motor energizationbetween approximate one-phase and substantially multiphase operation,and operator-controlled selective step switch means connected to saidenergizing means for controlling the latter in sequential steps.

2. A hoist control system comprising an alternating-current hoist motor,three-phase circuit means for energizing said motor for hoisting andlowering operations, a saturable reactor forming part of said circuitmeans and having a premagnetizlng control coil for varying the reactanceof said reactor, a current source, and operator-controlled selectivestep switch means connecting said source with said coil for regulatlngsaid reactance in sequential steps.

3. A control system comprising an alternatingcurrent motor having anenergizing circuit and a rotor circuit, multiphase control meansdisposed in said energizing circuit for selectively operating said motorin both directions, resistors arranged in said rotor circuit, means iorcontrolling said resistors, saturable reactance means connected in said,energizing circuit and including premagnetizing control windings,variable en'- crgizing means connected to said windings for changing themotor energization between approximate ope -phase and multiphaseoperation and master control means connected with said multiphasecontrol means, resistor control means and energizing means for operatingthem sequentially in a given relation to one another,

4. A control system comprising an alternatingcurrent motor, three-phasecircuit means for en ergizing said motor, variable impedance meansforming part of said circuit means. and having I an impedance rangesufficient for energizing said motor by substantially single phasevoltage and approximately balanced three-phase voltage, and selectivecontrol means under control by the operator and connected with saidimpedance means for progressively changing from said one to said othervoltage,

5. A control system comprising an alternating current motor, three-phaseenergizing circuit means connected to said motor, a saturable reactorhaving a main coil series connected to said motor in one phase of saidcircuit means and a control winding for varying the reactance of saidmain coil over a range extending from approxh mate single-phase toapproximately balanced three-phase energization of. said motor, a directcurrent circuit connected to said control winding and" comprising astepped resistor for sup lying variable energization to said controlcoil, and operator-actuated control means connected to said resistor iorprogressively changing the en ergization of said control coil in orderto change the energization of said motor.

6. A hoist control system comprising an alternoting-current hoist motor,three-phase energize lng circuit means connected to said motor :iorenergizing it for hoisting and lowering opera tions, a saturable reactorhaving a main coil series connected to said motor in one phase of saidcircuit means and a control winding for varying the reactance of saidmain coil, an energizing circuit connected to said control coil andcontaining a stepped resistance device for varying the energization ofsaid control coil to premagnetize said reactor between a minimum andmaximum magnetizatiomand a controllerhaving a plurality of selectivecontrol positions and containing contact means connected with saidcircuit means to control the latter and further contact means connectedwith said device tor controlling its resistance stepwise in accordancewith the selected controller position so that the reactance of saidreactor is stepwise decreased when changing the position of saidcontrollertowards high hoisting speed and increased when changingtowards high lowering speeds.

, 7. A hoist control system comprising an alternating-current hoistmotor, multiphase circuit means for energizing said motor for hoistingand lowering operations, a saturable reactor lorming part of saidcircuit means for varying the speed torque characteristic of said motorbetween predominant single-phase and multiphase operation, a mastercontroller for controlling said circuit means and said reactor in agiven relation to each other, and a separately controllable power brakeassociated with said motor and connected to said master controller so asto he operable only when Said circuit means and, reactor are set by saidcontroller ior zero speed and low speed lowering operation.

8. A hoist control system comprising a hoist motor, circuit means forenergizing said motor for hoisting and lowering operations, variableimpedance means connected with said motor for controlling its speed, amaster controller for controlling said circuit means and said hnpedancemeans in a given sequence, said controller having an on position and aplurality of hoisting and lowering positions, a power brake associatedwith said motor and connected to said master controller so as to iceeffective in the on position and ineffective in the hoisting positionor"- said controller, and separately aotuable control means operable bythe operator: ror releasing said brake when said master controller is insaid on" posi tion.

9. A hoist control system comprising an alternating-current hoist motorhaving a primary energizing circuit and a secondary rotor circuit, afriction hralre connected with said motor, electric means for energizingsaid brake normally connected to said primary circuit, a, mastercontroller having an off position and a plurality of hoisting andlowering positions and being connected with said primary and secondarycircuits for operating said motor at difierent hoisting and loweringspeeds, and separately actuated oporator-controlled contact means fordisconnecting said Toralre energizing means from said primary circuitand connecting it instead to said secondary circuit for speed-responsiveoperation, said contact means being electrically connected with saidmaster controller so as to he ineilective in hoisting and high-speedlowering positions of said controller.

10. A hoist control system comprising an alternating-current hoist motorergizing circuit means connected to said motor, a controller hav.

ing an on position and hoisting andlowering positions and beingconnected with said circuit means to control said motor in accordancewith said positions, a mechanical brake connected with said motor,current supply means for providing substantially constant energizationof said brake, speed responsive current supply means for energizing saidbrake in accordance with the motor speed, changeover relay interposedbetween said brake and said difierent supply means so as to normallyconnect said brake to said constant supply means, an operator-actuatedseparate switch for controlling said relay to change said brake over forenergization by said speed-responsive means, said switch being connectedwith said controller so as to be effective for starting a changeoveronly in said ofi position, said relay having a self-sealing contactconnected with said controller so as to permit maintaining said switchefi'ective only over a limited number of low-speed lowering positions ofsaid controller once said switch has been actuated in a proceeding onposition, whereby the system afiords stopping said motor at no load andlow-load lowering.

11. A motor control system comprising a wound rotor induction motorhaving multiplephase primary windings and a secondary resistancecircuit, multiple-phase alternating current supply means connected tosaid primary windings, primary voltage control means interposed betweensaid supply means and said motor for controlling the balance conditionof the primary phase voltage impressed on said windings, secondarycontrol means for controlling the resistance of said secondary circuit,and master control'means having a multiple number of selective controlpositions and being connected with saidprimary control means and saidsecondary control means for changing the degree of primary voltagebalance in a given sequential relation to each other. I

12. A motor control system comprising a wound rotor motor having primaryterminals and a secondary resistance circuit, multiphase current supplymeans connected with said terminals, voltage control means interposedbetween said supply means and said terminals for varying the balancecondition of the primary motor voltage, and operator-actuable selectivecontact means connected with said secondary circuit and with saidvoltage control means for causing the latter to unbalance the voltageacross said terminals when said secondary circuit is in at least oneselected resistance condition and causing said voltage control means tomaintainsaid voltage substantially balanced when said circuit is inanother selected condition.

13. A hoist control system comprising a, wound rotor motor havingprimary terminals and a secondary resistance circuit, multiphase currentsupply means connected with said terminals, reversing contactorsassociated with said supply means, unbalanceable voltage control meansinterposed between said supply means and said terminals, anoperator-actuated master controller connected with said contactors andsaid secondary circuit and with said control means for causing thelatter to unbalance the voltage across said terminals when saidcontactors and secondary circuit are set by said master controller forlow speed lowering operation of said motor.

14. A motor control system comprising an alternating current multiphasemotor, multiphase current supply means therefor, stepwise variablevoltage control means disposed between said supply means and said motorfor varying the voltage distribution of said motor over a stepped rangeincluding a substantially balanced and an unbalanced voltagedistribution, and a multiple point operator-actuated master controllerhaving sequential contact steps connected with said voltage controlmeans for causing them to change said voltage distribution in order'tochange the speed torque characteristic of said motor in correspondingsteps.

15. A hoist control system comprising, in combination, an alternatingcurrent hoist motor having standard connected primary windings, amultiphase circuit connected to said windings and containing variablecircuit means for unbalancing the voltage of said windings by varyingthe voltage of at least one of them between two values corresponding tolow speed high torque and low speed low torque characteristicsrespectively, and an operator actuable controller for stepwise varyingsaid circuit means between said values through selective intermediatestages corresponding to speed torque characteristics intermediate theaforesaid characteristics.

16. A hoist control system comprising, in combination, an alternatingcircuit hoist motor having standard connected primary windings, amultiphase circuit connected to said primaries and containing variablereactor means associated with a lesser number of phases for varying thevoltages of said primaries relative to each other through a rangeincluding a substantially balanced multiphase voltage distribution and agreatly unbalanced voltage distribution to obtain,

corresponding low speed high torque and low speed low torquecharacteristics respectively, and an operator actuable controller forvarying said circuit means through selective intermediate stagescorresponding to speed torque characteristics intermediate the aforesaidcharacteristics.

17. A hoist control system comprising an altemating-current hoist motor,three-phase current supply means connected to said motor, a variableimpedance device disposed in a lesser number of phases of said supplymeans for changing the energization of said motor through selectiveintermediate stages between substantially balanced three-phaseenergization and approximate single-phase energization, and means forcontrolling said supply means and impedance means so as to provideapproximate single-phase and intermediate energization at overhaulinglowering operations and substantially balanced three-phase energizationat hoisting operation of said motor.

18. A. hoist control system comprising an alternating current hoistmotor, three-phase current supply means connected to said motor,saturable reactor means associated with a lesser energization to saidmotor at overhauling lower-- ing operation.

WILLIAM R. WICKERHAM.

